The invention relates to balanced pressure valves of the kind comprising: a spring-loaded main piston, one face of which is acted upon directly, and the other, spring-loaded face of which is acted upon via two restrictors, located one behind the other, by the system pressure; a servo valve having a spring-loaded operating element by which the space between the two restrictors can be connected with exhaust when the system pressure exceeds the pressure set on the operating element, whereby, by means of the pressure differential acting on the main piston, the latter is displaced and a connection is established from a line carrying the system pressure to exchaust; and a switching valve by means of which a connection can be established from the line carrying the system pressure, via a further restrictor, to exhaust whereby the pressure valve can be switched over to pressureless circulation.
Such pressure valves serve as relief-valves for the setting of a specified system pressure. While directly controlled relief-valves find application for smaller flow quantities, balanced relief-valves are required for larger flow quantities. The servo valve is a directly controlled pressure valve, upon the operating member of which the system pressure acts. If and when the servo valve opens, then a pressure gradient arises between the two faces of the main piston, as a result of which the main piston is displaced and thereby allows excess fluid to flow away out of the system to the tank.
The balanced pressure valve includes additionally, in known manner, a switching valve via which the line carrying the system pressure can have a direct connection set up to the tank. By this means the pressure valve can for example be switched over to the pressureless starting-up of a pump or, at times of stoppage of a hydraulic installation, to pressureless circulation and hence low power loss.
If now, for example, through electromagnetic activation of the switching valve, the pressure valve is switched over to pressureless circulation, then the volume of fluid displaced by the main piston of the pressure valve is led via a restrictor to the switching valve and thus to the tank, so that the opening motion of the main piston s damped, and thereby very high and steep pressure differentials in the installation are avoided. The greater the throttle effect of the damping restrictor, the more powerfully the opening motion of the main piston is damped. On the other hand, however, there is also a constant, relatively low quantity of flow via the restrictor and the switching valve to the tank. The greater the throttle effect of the restrictor, the higher is the rise in pressure upstream from the restrictor, as a result of which the main piston tries to shut, so that the throttle losses in pressureless circulation are raised.
The problem which forms the basis of the present invention consists in configuring the above-mentioned pressure valve in such a way that, on switching to pressureless circulation, good damping of the main piston of the pressure valve is effected, and the throttle losses are lowered.